1. Field of the Invention
The present invention relates to locomotive air dryers and, more specifically, to an approach for optimizing purge cycling of an air dryer.
2. Description of the Related Art
Twin tower desiccant air dryers are used removing water vapor from compressed air in locomotive braking systems. In a twin tower system, one tower comprises a column of desiccant material and is used to absorb the water vapor in the inlet air stream by flowing wet air through the desiccant column. The desiccant absorbs the water vapor from the air stream so that the air discharged from the outlet of the air dryer is nominally dryer by an amount proportional to, among other variables, the amount of desiccant material in the column of the tower, the geometry of the column, and the air velocity in the column. Eventually, the first tower becomes saturated with water and is no longer effective in removing water from the inlet air stream.
When the active tower becomes saturated, the control system for the air dryer switches the inlet air stream to the other tower, which is nominally the same construction as the first tower, so that drying may continue. Concurrently, some of the dry air discharged from air dryer outlet is redirected through the first tower to atmosphere. This counter-flow of dry air removes the accumulated moisture from the desiccant column of the first tower and transports it to atmosphere. When the second tower eventually becomes saturated, the inlet air is switched back to the first tower and the second tower is purged, and the cycling between the two towers is repeated as needed. This counter-flow operation to remove moisture from a saturated column is referred to as a purge cycle and typically consumes 15-20% of the dry air discharged from the air dryer.
In an ideal locomotive air system having a constant air flow through the air dryer, the air dryer purge cycle and switching between the two desiccant columns can be done with a simple timer. In practice, however, the flow of air through the air dryer is never constant. For example, the air flow to the brake system can vary from very low flow rates needed to maintain the system against brake pipe leakage to very high air flows to recharge the train brake system after brake release. As a result, switching according to a fixed time schedule wastes energy and compressed air as a fixed-time purge cycle results in purge cycling before the desiccant in a particular column is fully saturated. While an air dryer system could include a flow meter or a humidity meter that more accurately determines when the purge cycle should be implemented, these technologies are expensive and often unreliable in the severe environment of a locomotive. Accordingly, there is a need for an inexpensive and reliable approach for determining when to perform the purge air cycling in a twin tower air dyer.